Virotherapy holds great promise for treating cancer. Oncolytic viruses, which aim to specifically infect and kill cancer cells, whether native and/or engineered, may be more efficacious and less toxic than alternative treatments, such as chemotherapy and radiation. In addition, oncolytic virus therapy that uses replication competent viruses is the only therapy known that can amplify the therapeutic at the pharmacologically desired site.
A key aspect of cancer therapy is to achieve a high rate of killing of cancer cells versus normal cells. Accomplishing this goal has been difficult for many reasons, including the wide array of cell types involved, the systemic dissemination of cancer cells due to metastases, and the narrow biological differences between normal and cancer cells. While progress has been made, much still needs to be done to improve upon current cancer therapies.
In the past, surgeons have tried to remove tumors surgically without substantially harming the patient. Even complete removal of a primary tumor does not ensure survival since earlier metastases to unknown sites in the body are left undetected. There is also some research suggesting that surgical intervention may enhance the growth of distant metastases due to removal of tumor cells producing angiogenesis inhibitors. Finally, in many cases, the tumor grows back at the original site after surgical removal. Radiation aims to selectively destroy the most rapidly proliferating cells at the expense of the others. However, tumor cells can escape radiation therapy either by becoming resistant or by being in a non-dividing state during treatment. In addition, radiation is not always selective in that many normal cells are actively dividing and killed by the treatment (cells in bone marrow, gastrointestinal cells, hair follicles, etc.).
Like radiation, chemotherapy is not completely selective and thus destroys many normal cells, and does not kill all tumor cells due to drug resistance and/or division state of the cell. Thus, chemotherapy and radiation therapies exploit a small differential sensitivity that exists between normal and cancer cells, giving them a narrow therapeutic index. A small therapeutic index is clearly an undesirable property of any modality to treat cancer. Therefore, novel cancer therapeutic approaches overcoming these limitations are desired.
One such novel approach is oncolytic virus therapy. Initially, replication-defective viruses carrying cytotoxic transgenes were utilized in attempts to treat cancer. However, they were found to be inefficient in transduction of tumors, inadequate spread within the tumor mass and not adequately selective toward cancers. To overcome this limitation, viruses were either modified to replicate selectively in tumor cells or viruses were discovered to have natural tumor-selective properties. These oncolytic viruses thus had the properties to replicate, spread, and kill tumor cells selectively through a tumor mass by locally injecting the virus or by systemically delivering the virus (FIG. 1).
Despite the early promise of this newly defined class of anti-cancer therapeutics, several limitations remain that may limit their use as a cancer therapeutic. Therefore, there is an ongoing need for novel oncolytic viruses that can be utilized for cancer therapy.